School has nothing about a mollusc (5)
I believe the answer is:
snail
'mollusc' is the definition.
(I know that snail is a type of gastropod)
'school has nothing about a' is the wordplay.
'school' becomes 'S'.
'has' is a charade indicator (letters next to each other).
'nothing' becomes 'nil' (eg nil is zero in football).
'about' means one lot of letters goes inside another.
'nil' going around 'a' is 'nail'.
's'+'nail'='SNAIL'
(Other definitions for snail that I've seen before include "Edible terrestrial mollusc" , "Slow mover" , "Not one of the fleet" , "Gastropod mollusc" , "Slow slithery animal" .)